This invention relates to a method and apparatus for separating air.
The most important method commercially for separating air is by rectification. A frequently used method of separating air by rectification includes steps of compressing a stream of air, purifying the resulting stream of compressed air by removing from it water vapor and carbon dioxide, and cooling the resulting purified stream of air by heat exchange with returning product streams to a temperature suitable for its rectification. The rectification is performed in a so-called "double rectification column" comprising a higher pressure and a lower pressure rectification column. Most if not all of the air is introduced into the higher pressure rectification column and is separated into oxygen-enriched liquid and nitrogen vapor. The nitrogen vapor is condensed. Part of the condensate is used as liquid reflux in the higher pressure column. Oxygen-enriched liquid is withdrawn from the bottom of the higher pressure column is sub-cooled and is introduced into an intermediate region of the lower pressure column through a throttling or pressure reducing valve. The oxygen-enriched liquid is separated into substantially pure oxygen and nitrogen products in the lower pressure column. These products are withdrawn from the lower pressure column and form the returning streams against which the incoming air is heat exchanged. Liquid reflux for the lower pressure column is provided by taking the remainder of the condensate from the higher pressure column, sub-cooling it, and passing it into the top of the lower pressure column through a throttling or pressure reduction valve.
Conventionally, the lower pressure column is operated with a pressure at its top in the range of 1 to 1.5 bar absolute. Liquid oxygen at the bottom of the lower pressure column is used to meet the condensation duty at the top of the higher pressure column. Accordingly, nitrogen vapor from the top of the higher pressure column is heat exchanged with liquid nitrogen in the bottom of the lower pressure column. Sufficient liquid oxygen is able to be evaporated thereby to meet the requirements of the lower pressure column for reboil and to enable a good yield of gaseous oxygen product to be achieved. The pressure at the top of the higher pressure column and hence the pressure to which the incoming air is compressed are arranged to be such that the temperature of the condensing nitrogen is a degree or two Kelvin higher than that of the boiling oxygen in the lower pressure column. In consequence of these relationships, it is not generally possible to operate the higher pressure column below a pressure of about 5.5 bar.
Improvements to the air separation process enabling the higher pressure column to be operated at a pressure below 5.5 bar have been proposed when the oxygen product is not of high purity. U.S. Pat. No. 4 410 343 discloses that when lower purity oxygen in it is required, rather than having the above-described link between the lower and higher pressure columns, air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate the oxygen product. The resulting condensed air is fed into both the higher pressure and lower pressure columns. A stream of oxygen-enriched liquid is withdrawn from the higher pressure column, is passed through a throttling valve and a part of it is used to perform the nitrogen condensing duty at the top of the higher pressure column.
U.S. Pat. No. 3,210,951 discloses a process for producing impure oxygen in which air is employed to boil oxygen in the bottom of the lower pressure column in order both to provide reboil for that column and to evaporate oxygen product. In this instance, however, oxygen-enriched liquid from an intermediate region of the lower pressure column is used to fulfil the duty of condensing nitrogen vapor produced in the higher pressure column.
Although the process is described in U.S. Pat. Nos. 4,410,343 and 3,210,951 make possible some measure of reduction in the ratio of the operating pressure of the higher pressure column to the operating pressure of the lower pressure column when the oxygen product is not pure, a further improvement would be particularly desirable. The present invention relates to methods and plants for separating impure oxygen from air which are intended to reduce the total power consumption.
According to the present invention there is provided a method of separating air, comprising separating nitrogen from a first stream of compressed air in a higher pressure rectification column, separating nitrogen from a second stream of compressed air in an intermediate pressure rectification column, introducing oxygen-enriched liquid air into a lower pressure rectification column, withdrawing from the lower pressure rectification column an impure oxygen product separated therein from the oxygen-enriched liquid air, and supplying liquid nitrogen reflux to each rectification column, wherein a part of the first air stream is condensed upstream of the higher pressure rectification column by indirect heat exchange with a first liquid stream withdrawn from mass exchange in the lower pressure rectification column, nitrogen taken from mass exchange in the higher pressure rectification column is condensed by indirect heat exchange with a second stream of liquid withdrawn from mass exchange in the lower rectification column, and a part of the second air stream is condensed upstream of the intermediate pressure rectification column by indirect heat exchange with a third stream of liquid withdrawn from mass exchange in the lower pressure rectification column, wherein the first, the second and third streams of liquid are all of a different composition from one another.
The invention also provides apparatus for separating air comprising a higher pressure rectification column for separating nitrogen from a first stream of compressed air, an intermediate pressure rectification column for separating nitrogen from a second stream of compressed air, a lower pressure rectification column for separating an impure oxygen product from oxygen-enriched liquid air, the lower pressure rectification column having an outlet for the impure oxygen product, at least one source of liquid nitrogen reflux for the rectification columns, a first condenser for condensing upstream of the higher pressure rectification column a part of the first stream of compressed air by indirect heat exchange with a first liquid having heat exchange passages communicating with at least one first mass exchange region of the lower pressure rectification column, a second condenser for condensing nitrogen from the higher pressure rectification column by indirect heat exchange with a second liquid having heat exchange passages communicating with at least one second mass exchange region of the lower pressure rectification column, and a third condenser for condensing a part of the second stream of compressed air by indirect heat exchange with a third liquid having heat exchange passages communicating with at least one third mass exchange region of the lower pressure rectification column, wherein the communication between the lower pressure rectification column and each of the condensers is such that in operation the first, second and third liquids are all able to have a different composition from one another.
By employing both the higher pressure rectification column and the intermediate pressure rectification column in the method according to the invention, it is possible to achieve in comparison with conventional processes a reduction in the proportion of the incoming air that has to be compressed to the operating pressure of the higher pressure rectification column or to an even higher pressure. The method according to the invention thus makes possible an overall saving in the power consumed in separating a given volume of air.
Preferably, a fourth stream of liquid is withdrawn from mass exchange in the lower pressure rectification column and is employed to condense nitrogen vapor taken from the intermediate pressure rectification column, the condensation being performed in a fourth condenser. It may alternatively be possible to condense nitrogen taken from the intermediate pressure rectification column by indirect heat exchange with an oxygen-enriched liquid air stream withdrawn from either the higher pressure rectification column or the intermediate pressure rectification column. Condensation of the nitrogen from the higher pressure and intermediate pressure rectification columns typically provides all the liquid nitrogen reflux or the rectification columns employed in the method according to the invention.
The second, third and, if employed, the fourth condensers are preferably each located in the lower pressure rectification column with there being mass exchange between ascending vapor and descending liquid taking place in sections of the column between respective pairs of condensers. The fourth condenser is preferably located above the third condenser with there being mass exchange between rising vapor and descending liquid therebetween. The first condenser is typically located outside the lower pressure rectification column or below the lowest mass exchange region therein. Each of the condensers is able to provide reboil to the lower pressure column. The opportunity thus arises with the method according to the invention to provide reboil at four separate regions of the lower pressure rectification column, thereby facilitating its operation at relatively high thermodynamic efficiency in comparison with the known processes described herein before.
Preferably, the impure oxygen product is withdrawn from the lower pressure column in liquid state. If this oxygen product is required at pressure, it is preferably vaporized by heat exchange with a third air stream at a higher pressure than the first and second air streams. The third air stream is typically at least partially condensed thereby and is preferably introduced into one or more of the rectification columns. For example, a part of the condensed third air stream may be introduced into the higher pressure rectification column and another part into the intermediate pressure rectification column. If desired, such introduction may be effected by premixing the respective part of the condensed third air stream with the first and second air streams, if desired, upstream of the first and third condensers respectively.
If none of the oxygen product is withdrawn from the lower pressure rectification column in liquid state, preferably a portion of that part of the first air stream that is condensed is introduced to the intermediate pressure rectification column.
If the lower pressure rectification column is operated at a pressure at its top of less than 1.5 bar, the oxygen-enriched liquid for separation therein is preferably taken in part from the higher pressure rectification column and in part from the intermediate pressure rectification column.
The respective air streams are preferably taken from one or more sources of compressed air that has been purified by removal of water vapor and carbon dioxide and cooled to a temperature suitable for its separation by rectification. If desired, a fourth stream of air may be formed by taking a part of the air being cooled and expanding it with the performance of external work. The fourth stream of air is preferably introduced into the lower pressure rectification column.
The rectification columns may effect liquid-vapor contact and hence mass exchange between liquid and vapor by using distillation trays or by using packing, for example structured packing. The term "mass exchange region" or "liquid-vapor contact region" as used herein refers in the case of distillation trays to a single distillation tray or in the case of packing to a section of packing.
It is to be understood that the operating pressure at the top of the intermediate pressure column is lower than the operating pressure at the top of the higher pressure rectification column but higher than the pressure at the top of the lower pressure rectification column.
The method and apparatus according to the invention are suitable for use in producing a main oxygen product containing from 80 to 97% by volume of oxygen.
In addition, it is possible to produce at a limited rate a higher purity oxygen product by employing a further liquid-vapor mass exchange region below the level of the lower pressure column from which the main oxygen product is taken, and withdrawing the higher purity product from a lower part of said further mass exchange region. For example, a 99.5% pure oxygen product can be so produced at a rate of up to 15% the rate at which the main oxygen product is taken.